1. Field of the Invention
The present invention relates to a motor driven lens having an automatic focusing device and a power zoom system, etc., and a camera system having such a motor driven lens.
This application is related to the commonly assigned applications U.S. Ser. Nos. 07/410,880, 07/652,038, 407,424, 07/414,477, 07/414,478, 07/670,733, 07/406,523, and 07/414,476, the disclosures of are which expressly incorporated by reference herein.
2. Description of Related Art
There are various known lens shutter type cameras having automatic focusing devices and power zoom lenses, in which zooming is effected by a motor. In a known lens shutter type of power zoom camera which has no interchangeable lens, the power zooming operation is controlled by a motor, a control means and a zooming switch, provided in a camera body. In a power zoom lens, the zoom motor must be stopped when the zooming lens group (i.e., variable lens and compensating lens) reaches an extreme position, beyond which the lens cannot be moved. At this point the power source is turned OFF to cut the supply of power to the zoom motor after a predetermined error checking time has elapsed from the point at which rotation of the zoom motor has been hindered. During the error checking time, the occurrence of an error or mal-operation is checked.
In the conventional camera, the error checking time is constant, independent of the number of revolutions of the motor. Accordingly, as motor overload increases, particularly when the motor is driven at high speed. Consequently, the motor and the driving system can be easily damaged. The same is true in a single lens reflex camera having an automatic focusing device in which a focusing lens group is driven by the motor. A lens group which is driven by a motor, such as a focusing lens group or a zooming lens group, will hereinafter be referred to as a motor driven lens.
Furthermore, it is necessary to detect the position of a focusing lens group and the position of a zooming lens group in order to obtain object distance data and focal length data, which are used to determine photographic requirements for taking a picture at optimum exposure.
In a known power zoom lens camera having an automatic focusing device, a detecting means for detecting the positions of the lens groups is, for example, formed by a code plate which detects absolute positions of the lens groups. For instance, in a conventional power zoom lens, the focal length is detected by a code plate which is secured to a zoom ring which rotates about or moves along the optical axis in accordance with the zooming operation. The code plate extends in the direction of movement of the zoom ring. A code reading member is provided on an immovable portion of the power zoom lens and is opposed to the code plate, so that the codes of the code plate are read by the code reading member to detect the focal length. The position of the focusing lens group can be similarly detected.
However, the detecting means made of the code plate can detect only the positions of the lens groups corresponding to switching points of the codes, but not the positions corresponding to the intermediate points between the switching points of the codes. Namely, only a rough detection (measurement) can be effected (i.e., a precise detection (measurement) cannot be effected).
It is also known to count the number of pulses (i.e., add or subtract a number in accordance with the direction of movement of the lens groups) outputted from a pulse generator in association with the rotation of a motor for driving the lens group, in order to more precisely detect the position of the lens group. In this detecting means, it is inevitable that an error in the number of pulses counted by the counter will occur. To avoid such an error, in the conventional camera, the pulse number is corrected when the lens groups reach an infinite extremity, a macro extremity, a wide angle extremity, and a telephoto extremity, etc.
However, if the lens drive is forced to a stop for some reason, for example, when the focusing ring is immovably held by an operator, the motor is stopped. As a result, the control means may determine that the lens group has arrived at an extremity of movement, so that the pulse number counted by the counter is corrected or the counter is reset. Furthermore, even if the lens is to be moved (i.e., power is being supplied to the lens), there is a possibility that the lens movement may be impeded by an external force, preventing movement of the lens group beyond the stopped position to the associated extremity.